Hydraulic actuator

ABSTRACT

A hydraulic actuator has two cylinder and piston motors of different area with the smaller piston connected to a piston rod and the larger piston capable of abutting the smaller piston. Preloaded springs bias the pistons toward each other. Servo fluid delivered between the pistons first moves both pistons a limited distance in one direction and then moves the smaller piston a larger distance in the other direction. The device may be doubleacting, in which case servo fluid may be applied to drive the pistons on the return stroke.

United States Patent [191 Harkrader May 21, 1974 HYDRAULIC ACTUATOR [75]Inventor: Ronald L. Harkrader, Saginaw,

Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: Oct. 27, 1972 [21] Appl. No.-: 301,671

[52] US. Cl 91/422, 91/468, 92/75, 92/130 [51] Int. C1..... F011) 7/02,F0lb 31/00 [58] Field of Search 92/75/69, 50, 130; 5 91/468, 422

[56] References Cited UNlTED STATES PATENTS 1,372,942 3/1921Constantinesco ..92/75 1,777,293 10/1930 Curtis et a1. 92/75 2,638,2895/1953 McKellar et al. 92/75 3,241,464 3/1966 Pierce, Jr 92/75 3,411,41011/1968 Westbury et al 92/75 Primary ExaminerPaul E. Maslousky Attorney,Agent, or Firm-Paul Fitzpatrick 5 7] ABSTRACT A hydraulic actuator hastwo cylinder and piston motors of different area with the smaller pistonconnected to a piston rod and the larger pistoncapable of abutting thesmaller pistonj Preloaded springs bias the pistons toward each other.Servo fluid delivered between the pistons first move's both pistons alimited distance in one direction and then moves the smaller piston alarger distance in the other direction. The device may be double-acting,in which case servo fluid may be applied to drive the pistons on thereturn stroke. e a

5 Claims, 4 Drawing Figures 1 HYDRAULIC ACTUATOR My invention relates tofluid pressure actuators and particularly to one which may be caused toexecute a particular routine of movement in response to fluid pressure.While the invention may be applicable to actuators energized by gasunder pressure, the preferred embodiment of the invention is directed toa device energized by substantially incompressible fluid.

The actuator was conceived in response to a requirement for a particularmode of operation of variable setting angle turbine nozzle vanes in agas turbine engine. The system calls for an actuator which shifts thevane angle a limited amount in one direction in response to increasingpressure supplied to the actuator and then, upon still furtherincreasing pressure, reverses the direction of movement of the vanes andmoves the vanes through a rather large angle. Specifically, the smallchanges in angle referred to may be for improving the response andefficiency of a gas turbine engine in normal operation, whereas the longtravel in the opposite direction may be for reversing the nozzle vanesto provide braking action from the turbine or to limit overspeed of theturbine.

The actuator may besingle-acting or double-acting, the provision of thedouble-acting characteristic involving some additions to the structure.The single-acting actuator is capable of effecting the movement desiredbut depends upon springs for its return movement when the fluid pressureis reduced or released. If there is substantial friction in the actuatedmechanism, this may prove undesirableytherefore, the actuator in itsdouble-acting configuration will be preferred in such cases. With adouble-acting actuator and with some form of feedback of the vane angleto the control device which supplies servo fluid to the actuator, thefriction can be overridden and vane angle can be precisely controllednotwithstanding friction.

The principal object of my invention is to provide a simple, reliable,and economical actuator to perform the cycle of operation describedabove.

A further object of the invention is to provide a hydraulic actuatorwhich is suited to the provision of relief valves to limit the actuatingforce exerted in either direction.

The nature of my invention and its advantages will be more clearlyapparent to those skilled in the art from the succeeding detaileddescription of the preferred embodiment of the invention.-

FIG. I is a plan view of an actuator embodying the invention.

FIG. 2 is a sectional view of the same taken on the plane indicated bythe line 2-2 in FIG. 1.

FIG. 3 is a sectional view taken on the plane indicated by the line 3-3in FIG. 1 illustrating a pressure relief valve.

FIG. 4 is a partial longitudinal sectional view of the relief valveillustrating a pilot valve therein.

Referring to FIGS. 1 and 2, the actuator depicted includes a body orhousing 2 which may be machined casting. The body includes a flange 3with bolt holes 4 for mounting the body on a device to be actuated suchas a turbine nozzle case. The body defines an internal cavity 6 ofcircular cross section part of which forms a first cylinder 7. Acylindrical boss 8 extending from the inner end of the cavity 6 definesa second cylinder 10 of considerably smaller diameter than cylinder 7.The body is bored at 11 coaxially with cylinders 7 and I0 to define abearing for a piston rod or actuating rod 12 which extends to theoutside of the body and serves to connect the actuator to the mechanismto be moved by it. A stop ring or washer 14 may engage the end of thehousing to limit movement of piston rod 12 into the body. This washer isimpinged between the body and a nut 15 threaded on the rod which may beturned to adjust the limit position. A second nut 16 acts as a jam nutto fix the adjustment. A suitable commercial seal 18 serves to preventleakage of hydraulic fluid from the housing along the rod 12.

The body defines two ports 19 and 20 for hydraulic actuating fluid orservo fluid. In a single-acting actuator, port 19 is the high pressureport and port 20 the low pressure. When the actuator is double-acting,however, either port may receive high pressure fluid from, or returndisplaced fluid to, the source of actuating fluid.

The large end of cavity 6 is closed by a cylinder head 22 retained by anexpanding snap ring 23. A plug 24 threaded into the head and sealed by agasket-26pmvides access to an adjustment to be described.

Port 19 communicates through a passage 27 (FIG. 3) with cavity 6 just tothe right, as illustrated, of a shoulder 28 at the inner end of cylinder7. It thus communicates withthe cylinders 7 and 10.

Piston rod.l2 is integral with a piston 30 reciprocable in cylinder 10and. the rod continues beyond the piston 30 to define a rod inner end3l. A thimble 32 which is swaged to the rod end 31 abuts a flange 33 andthus is connected to the piston rod 12. Thimble 32 includes a flange 34which serves as one abutment for a preloaded compression spring 35, theother end of which engages the end of chamber 6'. Spring 35 thus urgesthe piston rod 12 into the body. A cup-shaped piston 36 having pistonrings 38 is reciprocable to a limited extent in the cylinder 7. Thepiston 36 includes a central boss 39 which is slidable within thethimble'32. A vent 41 in the thimble prevents fluid blockage. The skirtof piston 36 is slidable within the cylinder head 22. Piston 36 isbiasedinto the cylinder 7 by a preloaded compression spring 40 whichengages between the cylinder head 22 and a flange 42 of the piston.

Thus, both piston 30 and piston 36 are urged into the cylinder by thesprings 35 and 40, respectively,;and are thus urged toward each other.The pistons are normally in contact, with the rod inner end 31 engagingan adjustable stop 43 having a stem 44 threaded through the boss 39.Assuming that piston 36 is normally seated against'the shoulder 28,adjustment of stem 44 determines the rest position of piston rod 12.This adjustment may be locked by a jam nut 46. It is accessible byremoving plug 24. Piston 36 will separate at stop 43 from piston ,rod 12when it travels to its stroke limit against head 22 and stop 14 engagesthe housing 2.

Fluid is supplied to the outer faces of the pistons; that is, the leftface of piston 36 and the right face of piston 30 as illustrated, fromport 20 through passages including a passage 47 extending from port 20into cavity 6 and a passage 48 which intersects a passage 50 enteringthe closed end of cylinder 10. Passage 50 also provides for drainage offluid from the seal l8. V

Passage 47 is intersected by a stepped bore 51 in the body 2, at rightangles to the cylinders, which provides a housing for a relief valve 49including a spool 52 (FIGS. 3 and 4). The passage 27 from port 19intersects one end of the bore 51 at a port 54. The passage 47communicates with a zone 55 of the bore between lands 56 and 58- on therelief valve spool. The valve spool is biased to the positionillustrated in FIG. 3 against a shoulder in the bore by a preloadedcompression spring 59 one endof which abuts a cap 60 which closes theopen end of bore 51. This cap is retained by a snap ring and sealed byan G-ring. Passage 27 communicates also with the other end of valvespool 52 through a restricted passage 62 which terminates near the cap60.

As stated, the relief valve 52 is of a pilot-operated type which dependsupon a pilot valve to unbalance the pressure on the two ends of thespool when the pressure to be limited reaches the selected maximumlevel. Referring to FIG. 4, valve spool 52 defines an internal chamber63 within which is mounted a compression spring 64 which encloses anabutment 66 and bears against a shoulder on the abutment. Abutment 66urges a ball 67, which is a movable valve member, against an annularseat 68 threaded into the outer end of the spool by too high pressure,the flow from chamber 71 causes a pressure drop through passage 62,unbalancing the pressures on the ends of spool 52. The excess ofpressure from port 19 and passage 27 on the iower end, as illustrated inHO. 3, will move the spool upwardly to provide a connection betweenpassage 27 and passage 47 toallow the servo fluid to by-pass the pistonand cylinder assembly. This pressure level is adjusted by the positionof'the seat 68, which is set by shims 73 between the hexagonal head ofthe seat and the valve spool. 52. I

The relief valve 49 is provided whether the actuator is single-actingor'double-acting, since in the singleact'ing form theport I9 is thesupply or high pressure port. When the actuator is double-acting andthere may be high pressure at the port 20, a second relief valve 74 isprovided, preferably within the piston rod 12, 31.

Referring to FIG. 2, the piston rod has a longitudinal bore 75 enteredfrom within cylinder by a crossbore 76 and discharging at the other faceof the piston 30 into cylinder 7'through a cross-bore 78. Bore 75 de--fines'a seat fora valve ball 79 urged against it by a preloadedcompression spring 80. Spring 80 is loaded by an abutment 82 threadedinto the end portion 31 of the piston rod. Shifting the abutment changesthe load of spring 80 and thereby the pressure at which the reliefvalve-ball 79 unseats to discharge fluid fronvport through passages 48and 50, valve 74, and passage 27 into port 19.

Considering now the operation of the device, assuming that there isno'fluid pressure or that the pressures supplied to the portsl9 and 20are equal, the parts wil brain the position illustrated in FIG. 2.Spring 40 has a higher preload than spring 35 and thus forces piston 36against shoulder 28 and biases piston rod 12 to its position of restagainst theopposition of spring 35. This might, for example, move theturbine nozzle vanes to their engine idle or starting position. If thepressure in function of pressure difference between inlets l9 and 4 port19 communicated to the space between the pistons becomes higher thanthat at the supply to port 20, this force acting on the differential ofarea between pistons 36'and 30 tends to move the rod 12 inward. No

movement occurs until the pressure differential becomes great enough toovercome the springforce differential between springs 40 and 35. As thepressure increases above this point, the balanced position of thepistons with relation to spring force gradually shifts to the left asillustrated, and the rod '12 is moved inwardly through a range ofpositions, perhaps to a nozzle minimal area or economy position at whichthe stop 14 engages the body. lt would be possible to have the stopconstituted by engagement of piston 36 with the cylinder head, but thearrangement shown is preferred, particularly in view of the facility ofadjustment. If the pressure between the pistons. continues to increase,this pressure acting on the area of piston 30 moves the rod 12 outwardlyor'to the right as shown and it may move, for example, until the piston30 engages the inner end of cylinder 10. This outward movement may beutilized for intermediate settings, if desired. Full outward'travel willmove the turbine nozzle vanes to their reverse position. This movementmay determine position by balancing pressure against the resistance ofspring 35, but

it may be desired only to achieve full travel outward of rod 12 bysupplying high pressure to port 19. With a single-acting actuator, ifthe pressure is then gradually reduced, the operation takes-placereversely, with the springs moving the rod as the pressure decreases.

If, however, it is desired to supplement the spring force by hydraulicpressure to overcome possible friction in the turbine nozzle, thedouble-acting construction is employed including the relief valve 74. Inthis case, if the movement of rod 12 does not follow a control input,the hydraulic pressure can be reversed through a position feedback topositively drive thepiston 30 inward initially and finally exertedthrough passage 47 against the outer face of piston 36 to drive it backto its shoulder 28, carrying piston 30 with it. If the overbalance ofpressure becomes too great, the relief valve 74 can operate to reliveit. Relief valve 74 is redundant with a single-acting system but does noharm.

The operating characteristics of the actuator; that is, itscharacteristic of displacement of the rod 12 as a 20, in the absence ofexternal forces, depends upon several factors. These are the effectiveareas of the pistons 30 and 36 and the preloadsand spring. rates ofsprings 35 and 40. These may also be expressed as ratios of theseparameters. A wide variety of characteristics are available, and anyparticular. one may be adopted by suitable selection of the parameters.

It should be clear from the foregoing to those skilled in the art that Ihave devised an actuator that is of simple and reliable construction andvery well adapted to operate either as a single-acting or adouble-acting hydraulic piston and cylinder device and which has ahighly desirable movement characteristic for application to adjustmentof turbine nozzles.

The detailed description of the preferred embodiment of the inventionfor the purpose of explaining the principles thereof is not to beconsidered as limiting or restricting the invention, since manymodifications may be made by the exercise of skill in the art.

I claim:

1. A hydraulic actuator comprising, in combination, a body defining afirst cylinder and a second cylinder coaxial with and communicating withthe first, a first piston reciprocable in the first cylinder, a secondpiston reciprocable in the second cylinder, the second piston being ofless area than the first, means on the pistons normally abutting, afirst spring biasing the first piston toward the second piston, a secondspring biasing the second piston toward the first piston, the firstspring having a higher preload than the second spring, the ratio ofspring force to piston area of the first cylinder being lower than thatof the second cylinder, a first servo fluid connection communicatingwith the confronting faces of the pistons, anda second servo fluidconnection communicating with the remote faces of both pistons; and anactuating rod connected to the second piston and extending from thebody.

2. A hydraulic actuator comprising, in combination, a body defining afirst cylinder and a second cylinder coaxial with andcommunicating withthe first, a first piston reciprocable in the first cylinder, a secondpiston reciprocable in the second cylinder, the second piston being ofless area than the first, axially adjustable means on the pistonsnormally abutting, a first spring biasing the first piston toward thesecond piston, a second spring biasing the second piston toward thefirst piston, the first spring having a higher preload than the secondspring, the ratio of spring force to piston area of the first cylinderbeing lower than that of the second cylinder, a high pressure servofluid connection communicating with the confronting faces of thepistons, and a low pressure servo fluid connection communicating withthe remote faces of both pistons; and an actuating rod connected to thesecond piston and extending from the body.

first spring having a higher preload than the second spring, the ratioof spring force to piston area of the first cylinder beinglower thanthat of the secondcylinder, a first servo fluid connection communicatingwith the confronting faces of the pistons, and a second servo fluidconnection communicating with the remote faces of both pistons; and anacutating rod connected to the second piston and extending from thebody.

4. A'double-acting hydraulic actuator comprising, in combination, a bodydefining a first cylinder and a second cylinder coaxial with andcommunicating with the first, a first piston reciprocable in the firstcylinder, a second piston reciprocable in the second cylinder, thesecond piston being of less area than the first, means on the pistonsnormally abutting, a first spring biasing the first piston toward thesecond piston, a second spring biasing the second piston toward thefirst piston, the first spring having a higher preload than the secondspring, the ratio of spring force to piston area of the first cylinderbeing lower than that of the second cylinder, a first servo fluidconnection communicating with the confronting faces of the pistons, anda second servo fluid connection communicating with the remote faces ofboth pistons, a first pressure-limiting relief valve operative to bleedservo fluid from the first connection to the second, a secondpressure-limiting relief valve effective to bleed servo fluid from thesecond connection to the first connection; and an actuating rodconnected to the second piston and extending from the body.

5. A double-acting hydraulic actuator comprising, in combination, a bodydefining a first cylinder and a second cylinder coaxial with andcommunicating with the first, a first piston reciprocable in the firstcylinder, a second piston reciprocable in the second cylinder, thesecond piston being of less area than the first, axially adjustablemeans on the pistons normally abutting, a first spring biasing the firstpiston toward the second piston, 21 second spring biasing the secondpiston toward the first piston, the first spring having 'a higherpreload than the second spring, the ratio of spring force to piston areaof the first cylinder being lower than that of the second cylinder, afirst servo fluid connection communicating with the confronting faces ofthe pistons, and a second servo fluid connection communicating with theremote faces of both pistons, a first pressure-limiting relief valveoperative to bleed servo fluid from the first connection to the second,a second pressure-limiting relief valve mounted in the second pistoneffective to bleed servo fluid from the second connection to the firstconnection; an actuating rod connected to the second piston andextending from the body; and

limit stop means effective to limit movement of the actuating rod ateach end of its travel.

1. A hydraulic actuator comprising, in combination, a body defining afirst cylinder and a second cylinder coaxial with and communicating withthe first, a first piston reciprocable in the first cylinder, a secondpiston reciprocable in the second cylinder, the second piston being ofless area than the first, means on the pistons normally abutting, afirst spring biasing the first piston toward the second piston, a secondspring biasing the second piston toward the first piston, the firstspring having a higher preload than the second spring, the ratio ofspring force to piston area of the first cylinder being lower than thatof the second cylinder, a first servo fluid connection communicatingwith the confronting faces of the pistons, and a second servo fluidconnection communicating with the remote faces of both pistons; and anactuating rod connected to the second piston and extending from thebody.
 2. A hydraulic actuator cOmprising, in combination, a bodydefining a first cylinder and a second cylinder coaxial with andcommunicating with the first, a first piston reciprocable in the firstcylinder, a second piston reciprocable in the second cylinder, thesecond piston being of less area than the first, axially adjustablemeans on the pistons normally abutting, a first spring biasing the firstpiston toward the second piston, a second spring biasing the secondpiston toward the first piston, the first spring having a higher preloadthan the second spring, the ratio of spring force to piston area of thefirst cylinder being lower than that of the second cylinder, a highpressure servo fluid connection communicating with the confronting facesof the pistons, and a low pressure servo fluid connection communicatingwith the remote faces of both pistons; and an actuating rod connected tothe second piston and extending from the body.
 3. A double-actinghydraulic actuator comprising, in combination, a body defining a firstcylinder and a second cylinder coaxial with and communicating with thefirst, a first piston reciprocable in the first cylinder, a secondpiston reciprocable in the second cylinder, the second piston being ofless area than the first, means on the pistons normally abutting, afirst spring biasing the first piston toward the second piston, a secondspring biasing the second piston toward the first piston, the firstspring having a higher preload than the second spring, the ratio ofspring force to piston area of the first cylinder being lower than thatof the second cylinder, a first servo fluid connection communicatingwith the confronting faces of the pistons, and a second servo fluidconnection communicating with the remote faces of both pistons; and anacutating rod connected to the second piston and extending from thebody.
 4. A double-acting hydraulic actuator comprising, in combination,a body defining a first cylinder and a second cylinder coaxial with andcommunicating with the first, a first piston reciprocable in the firstcylinder, a second piston reciprocable in the second cylinder, thesecond piston being of less area than the first, means on the pistonsnormally abutting, a first spring biasing the first piston toward thesecond piston, a second spring biasing the second piston toward thefirst piston, the first spring having a higher preload than the secondspring, the ratio of spring force to piston area of the first cylinderbeing lower than that of the second cylinder, a first servo fluidconnection communicating with the confronting faces of the pistons, anda second servo fluid connection communicating with the remote faces ofboth pistons, a first pressure-limiting relief valve operative to bleedservo fluid from the first connection to the second, a secondpressure-limiting relief valve effective to bleed servo fluid from thesecond connection to the first connection; and an actuating rodconnected to the second piston and extending from the body.
 5. Adouble-acting hydraulic actuator comprising, in combination, a bodydefining a first cylinder and a second cylinder coaxial with andcommunicating with the first, a first piston reciprocable in the firstcylinder, a second piston reciprocable in the second cylinder, thesecond piston being of less area than the first, axially adjustablemeans on the pistons normally abutting, a first spring biasing the firstpiston toward the second piston, a second spring biasing the secondpiston toward the first piston, the first spring having a higher preloadthan the second spring, the ratio of spring force to piston area of thefirst cylinder being lower than that of the second cylinder, a firstservo fluid connection communicating with the confronting faces of thepistons, and a second servo fluid connection communicating with theremote faces of both pistons, a first pressure-limiting relief valveoperative to bleed servo fluid from the first connection to the second,a second pressure-limiting relief valve mounted in the second pistoneffective to bleed servo fluid from the second connection to the firstconnection; an actuating rod connected to the second piston andextending from the body; and limit stop means effective to limitmovement of the actuating rod at each end of its travel.